The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air-fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug to generate power transferable to a crankshaft. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection to generate power transferable to a crankshaft. Combustion for both a gasoline engine and a diesel engine involves premixed or diffusion flames controlled by fluid mechanics.
An engine configured for SI operation including direct fuel-injection can operate in a controlled auto-ignition combustion mode, also referred to as homogeneous charge compression ignition (HCCI) under predetermined speed/load operating conditions. The controlled auto-ignition combustion mode includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the controlled auto-ignition combustion mode has an intake air/fuel charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. The controlled auto-ignition combustion results in a distributed kinetically-controlled combustion with the engine operating at a dilute air-fuel mixture, i.e., a mixture that is lean of stoichiometry. This engine operation results in relatively low peak combustion temperatures and low NOx emissions. The homogeneous air-fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
In engine operation, the engine air flow is controlled by selectively adjusting position of the throttle valve and adjusting opening and closing of intake valves and exhaust valves that control airflow into each combustion chamber. An engine can be equipped with a variable valve actuation (VVA) system that includes cam phasing and a selectable multi-step valve lift, e.g., multiple cam lobes which provide two or more valve lift profiles to control openings and closings of the intake valves and exhaust valves. A change in the valve lift profile of the multi-step valve lift mechanism is a discrete change.
When an engine operates in the controlled auto-ignition combustion mode, engine control includes operating at a lean air-fuel ratio with the throttle wide open to minimize engine pumping losses. When an engine operates in the spark-ignition combustion mode, the engine control includes operating at a stoichiometric air-fuel ratio with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air-fuel ratio. Engine output power is controlled by controlling fuel flow to the engine.
In an engine selectively operative in one of the spark-ignition combustion mode and the controlled auto-ignition combustion mode, transitioning between combustion modes can be complex. The engine controller must coordinate multiple actuators in order to provide the desired air-fuel ratio for the different modes. During a HCCI to SI transition, switching of the multi-step valve lift occurs nearly instantaneously and adjusting the cam phasings and the throttle includes slower dynamics. Engine torque disturbances and misfires may occur during combustion mode transitions when switching of the multi-step valve lift and adjusting cam phasing of the variable valve actuation system are not properly managed.